Known techniques used to select and maintain laser resonator frequency of coupled resonators require that a portion of the output beam from each resonator be combined and interfered. The resulting interference pattern is a function of three parameters, the frequency of the beams as well as the relative phase and coherence therebetween. As is well known, differences in coherence and phase between the two beams Will alter the interference pattern, as will differences in frequency. The techniques which rely on beam interference to maintain optically coupled resonators must account for the effects of these other parameters. The intensity of the beams as well as such parameters as the coupling path length must be determined. Consequently, these known techniques are characterized by undesirably elaborate apparatus and additional measurements.
It would be advantageous to have a method and apparatus for maintaining laser resonators at a selected frequency which would not be effected by variations in beam phase and coherence. The present invention is drawn toward such a method and apparatus.